Glass filaments or fibers are typically formed by flowing molten glass through a large number of closely spaced orifices or tips located in the bottom of a heated container. Such containers, referred to in the art as bushings, may have as many as 2,000, 4,000 or even up to 6,000 individual glass fiber tips. The molten glass which flows through these tips must be cooled in a controlled fashion so that the glass fibers which are ultimately formed will have substantially uniform diameters.
In the past, a plurality of generally solid, planar, elongated elements, known as cooling fins were placed beneath the bushing tip plate and extended across its width. The fins were oriented and spaced so that usually one to two parallel rows of tips could easily be accommodated between them. This allowed the immediate cooling of the glass fibers as they emerged from the tips or orifices of the bushing. Upon their discharge into the atmosphere, the individual streams of glass would neck down, as determined by their viscosity and surface tension characteristics, to form a cone-like body of glass from which the fiber was drawn or attenuated. The term "cone" as used herein designates the shape assumed by the molten glass in the zone immediately below the tip or orifice from which the glass emerges. In a mathematical sense, this shape might be more accurately visualized as that of a decaying exponential. Observations and experiments indicated that heat losses from the cone were affected by radiative heat transport as well as by convection. It is believed that most radiation losses occur in the region immediately below the tip exit, in other words, at the base of the cone where the glass has its brightest glow and highest temperature. Accordingly, when the fins were placed near the glass cones emerging from the tips, heat transfer by way of radiation could be controlled and manipulated.
The heat radiated to these usually solid flat metal cooling fins of the prior art was conducted along them to a header block where one end of the fin was attached, usually by welding or brazing. The header block was typically provided with flow channels through which a coolant such as ordinary plant process water passed. This kept the fins at a relatively low temperature near their junction with the header block. The free end of the fins, on the other hand, was usually substantially warmer than the one in contact with the header block. This gave rise to a large front-to-back temperature variation or gradient along the surface of the fin.
With the advent of longer and wider bushing assemblies having as many as 6,000 or more individual tips, heat removal has become more difficult. Cooling fins have been made longer so as to extend across the width of these larger bushings. This increase in fin length has resulted in even higher surface temperatures at the free end of the fins than heretofore seen in industrial applications. This has led to excessive oxidation of the fin surface and the distortion of the free end of the fins which has resulted in a shorter life of the fin cooler assemblies. In industrial processes, as many as 3 to 8 fin cooler assemblies each having anywhere from 9 to 16 individual fins are required for each bushing assembly.
Fine textile quality fiber glass strand products typically require especially higher levels of product uniformity than the strand products used to produce rovings for resin reinforcement purposes. Textile glass fiber strands are used to manufacture cloth for use as a reinforcing laminate employed in electrical printed circuit boards or for manufacturing decorative fabrics. The use of solid fin coolers did not provide the desired uniformity to textile fibers in many instances or on any kind of a predictable basis. Furthermore, since the cooling capabilities of the solid fins are limited, forming tensions for textile fibers tended to be high and the glass throughput from the bushings relatively low.
Attempts have been made to provide fin cooler assemblies in which the individual fins contained hollow channels or passages so that plant cooling water could be passed through them to help maintain front-to-back temperature uniformity. Exemplary prior art references which disclose such fins can be found in U.S. Pat. Nos. 3,251,665 (Bour); 3,695,858 (Russell); 3,746,525 (Kasuga, et al.); and 4,824,457 (Jensen). Several problems tended to be associated with these earlier attempts at developing water-cooled fins, however.
First, the flow of ordinary plant process water through the fins resulted in fin surface temperatures which were unacceptably low, i.e, temperatures in the range of 70.degree. F. to 100.degree. F. on some portions of the fins. These low temperatures caused glass volatiles such as boron oxide, sodium borate, and hydrofluoric acid which are present in the moist environment near the tip plate of the bushing to condense out onto the surface of the fins thereby causing unacceptably high rates of corrosion that considerably reduced the life of the fin. Also, localized contaminant buildup on the surface of the fins altered their local radiative heat transfer characteristics thereby creating non-uniformity in the diameters of glass fibers produced from the same rows of tips. These low fin temperatures also allowed the glass volatiles to mix with water in the moist environment immediately adjacent to the bushing which resulted in the formation of corrosive acids that further reduced fin life. Secondly, the use of ordinary plant process cooling water or ethylene glycol solutions which were not completely free of entrained solid particles and other contaminants created additional problems. As this water continuously flowed through the relatively small passageways in the fins, the passages often tended to become plugged, blocked or occluded. This blockage often rendered the fin inoperative. Due to the presence of heat being constantly radiated onto the surface of the fin from the molten glass cones and the bushing tip plate as well, this flow blockage may be accompanied by a localized increase in coolant pressure due to boiling and the subsequent vaporization of the coolant inside the fin. This, in turn, could lead to mechanical distortion or, under extreme circumstances, localized rupture of the fin itself.
Thus, there exists a need in glass fiber-forming to consistently maintain the surface temperature of the fin cooler assemblies at a sufficiently high enough temperature to prevent the condensation of glass volatiles onto the surface of the fin thereby degrading fin efficiency and shortening fin life. There also exists a need to maintain a substantially uniform axial temperature distribution along the length of each fin. There also exists a need to accomplish these above-mentioned objectives in a practical manner and on an industrial scale. There also exists the need to maintain the continuous flow of coolant throughout the fin cooler system in the event of an emergency such as an interruption in coolant flow so as to prevent any subsequent fin distortion or possible localized rupturing which may occur. There also exists a need to provide a means for automatically monitoring and controlling such a system on an industrial scale.